1. Field of the Invention
The present invention relates to thermal conductivity detectors used in gas chromatography and other methods for analysis and measurement of vaporous substances.
2. Description of the Related Art
The thermal conductivity detectors (often described by the initials TCD) follow differential approach and use one, two or four heated elements set in cavities with isothermal walls, through which sample and reference gas can flow. Common elements are thermistors (glass-coated beads of metal oxides) and hotwires (straight or spiral filaments fabricated from tungsten or nickel). Because their utilization requires proper installation, usually these elements are spot welded on the leads of replaceable sensors set in cavities bored into a massive metal block with constant temperature.
Owing to their sizable resistance (about 500 ohm in action) and temperature coefficient of resistivity, the thermistors have very good sensitivity despite their relatively thick coating. They are compact and suit the low volume cavities requisite for capillary gas chromatography. However, the selection of matched pairs of thermistors for the widely used Wheatstone bridge is rather difficult. Additionally, the thermistors should not be used in hydrogen atmosphere and can operate only below 100° C.
The hotwires cover the higher temperatures needed to analyze samples with boiling points of up to 250° C., but have moderate sensitivity due to their lower resistances (10 to 70 ohm at 20° C.) and temperature coefficients of resistivity. Since the hotwires wear out relatively fast, experiments were made to insulate them with PTFE (polytetrafluoroethylene, known under the trademark Teflon® of DuPont®), however unsatisfactory poor sensitivities were obtained using filaments with low resistances and thick coatings.
The hotwires are easier to match and also allow miniaturization, but quite large cavities are necessary for the widespread replaceable sensors with spiral filaments. The performance of these sensors can be improved using elements with resistance up to 800-1000 ohm and designs, at which the filaments heated by constant electric current have maximum temperature, however the spirals of increased sizes require cavities exceeding the volume suitable for capillary gas chromatography.
What is needed for improving the conventional thermal conductivity detectors (TCD) are highly sensitive, stable in wide enough temperature range, matchable and compact elements and relevant low volume and fast responding TCD sensors with optimized performance.